Doctors are failing to give an accurate cause of death for one in four patients, according to the results of a pilot scheme aimed at improving scrutiny of doctors in England and Wales.

The results suggest that doctors may blame the wrong type of disease in as many as one in 10 cases.

The results suggest that families may often be told the wrong cause of death for a relative. It also means the NHS does not have an accurate picture of why people have died, which could have implications for allocation of services and health policy.

Moves designed to give grieving families more information and reassurance on what killed their relatives, and to improve care and the monitoring of public health trends are to be introduced across England and Wales in April 2014 when a 1,000-strong body of medical examiners starts double-checking death certificates.

Alan Fletcher, leading the pilot scheme for the new system in Sheffield, where a quarter of more than 8,000 certificates studied over four years fell short, said doctors were not routinely making disastrous mistakes or deliberate falsifications. "I don't believe there is someone of murderous intent patrolling hospital corridors. It is rare to get something off the wall. It is a question of precision."

Faults stemmed from doctors not "reading" the full stories behind patients' fatal illnesses, he said.

For instance, if a person who died of pneumonia also had advanced lung cancer, it was the cancer that caused the pneumonia and was ultimately the cause of death. Similarly, a person who was said to have died from pneumonia, might also have been old, frail, unable to get out of bed and had dementia, Fletcher said. As such, it was more accurate to say they had died from old age, frailty and dementia.

Doctors had not always understood when they must refer matters to a coroner, he added.

"There was a failure to appreciate a cause of death that might be unnatural. Take pulmonary fibrosis – a disease that might be acquired industrially. This is an area of mining and steelworks. The registrar would be required by law to refer this to a coroner.

"Doctors receive little or no training in death certification or coroner referral."

Fletcher said he was confident the medical examiner system would ensure that relevant cases would always be referred to a coroner, and that medical certificates were as accurate as possible.

He said there was no formal checklist for deciding a person's cause of death.

"It is more helpful to read the records as you might read a story. I would look at the name, age, start of the last illness, admission to hospital or the last visits to or from a GP. Was the working diagnosis correct? Were investigations done properly and acted on where necessary? I am looking for red flags; for instance, falls, has a patient's condition changed or a concern from relatives been recorded? I am even more careful if the patient had no relatives, because then they have no advocate."

He said there could be a spectrum of certainty. "There is a world of difference between citing a known precise cause of death for someone who has been in hospital for a week or two and undergone lots of tests, and Mr Smith who leaves his house in the morning and collapses on his way to the bus stop."

The new system is being introduced partly as a result of an inquiry into how the GP Harold Shipman managed to kill a suspected 260 people without authorities detecting his actions. Shipman, from Hyde, in Greater Manchester, was convicted and sentenced to life imprisonment for murdering 15 patients.

Fletcher said he did not believe changes would automatically prevent another such case, "although they might detect him or her more quickly". Changes would flag up clinical concerns, even where matters were not referred to a coroner.

"Let's say there is someone who has died from a very natural illness or disease, but some doses of medication were missed or observations were not recorded or acted on in a timely matter, I will discuss that with the coroner even if it might have no part in causing death, but then pass the concern to clinical governance systems if the coroner decides not to investigate.

"It is uncommon to come across something that is jaw-dropping. Catastrophic mistakes are usually recognised and reported but subtle things that are not immediately apparent may have a bearing for clinical governance."

Feedback from doctors had been "almost universally positive", he said. Relatives appreciated the accuracy and explanation of medical terminology "as well as the reassurance that someone independent has taken a look and asked if there were any concerns".

The public health minister Anne Milton said the reforms "will improve the accuracy and robustness of information we record on the causes of death. This is not only important for families but will help with public health surveillance and local health service planning. Critically it will also help with the early detection of poor-quality care."